\begin{tikzpicture}[>=Stealth, scale=0.8]
    \draw [->] (-1, 0) -- (4.5, 0) node[anchor=west] {$x$};
    \draw [->] (0, -1) -- (0, 4) node[anchor=east] {$y$};
    \node at (-0.3, -0.3) {$O$};

    \coordinate (O) at (0, 0);
    \coordinate (Z) at (3.5, 2.8);
    \draw[->, thick] (O) -- (Z);
    \coordinate (R) at (3.5, 0);
    \draw[dashed] (R) -- (Z);
    \draw [->] (1.0, 0) arc (0:39:1.0);
    \node at (1.2, 0.5) {$\theta$};
    \node at (1.8, -0.3) {$a$};
    \node at (3.8, 1.3) {$b$};
    \node at (1.4, 1.6) {$r$};
    \node at (4.3, 3.1) {$Z:\; a + b\,i$};
\end{tikzpicture}
